The present invention relates to a process for producing membranes for electroacoustic transducers, which comprise a core layer comprising poly(meth)acrylimide foam and at least one covering layer, and to diaphragms for electroacoustic transducers.
Electroacoustic transducers or loudspeakers are devices which are able to convert electric alternating currents in the sound frequency range into audible sound. These devices are widely known in the prior art and are described, for example, in U.S. Pat. No. 4,928,312, DE-A 30 36 876 and DE-A 22 25 710.
The production of these loudspeakers requires diaphragms which have to meet numerous conditions. Thus, the weight of the diaphragm should be as low as possible, while its strength should meet relatively high requirements so that the diaphragms behave as completely stiff cylinders even at high frequencies.
Thus, for example, EP-A-0 087 177 describes a diaphragm which comprises a layer of poly(meth)acrylimide foam. In this document, it is stated that the poly(meth)acrylimide-containing layer can be provided with a covering layer. This covering layer is applied at room temperature by means of adhesive to ensure that the density of the core layer remains as low as possible. According to EP-A-0 087 277, the product of density and modulus of elasticity should be as small as possible, since this factor is a measure of the quality of the diaphragm.
The European patent application EP-A-0 126 841 discloses a loudspeaker diaphragm comprising two paper layers which are joined to one another over their entire area by an intermediate layer of plastic. The sandwich structure is produced by placing a foamed thermoplastic between the paper layers and compressing this composite between heated press platens until the plastic intermediate layer has a thickness which is from about 1 to 2 times the thickness of one of the paper layers.
Loudspeaker diaphragms can be provided with covering layers for many reasons. These include, inter alia, increasing the strength of the diaphragm, or aesthetic reasons. However, the process proposed in EP-A-0 087 177 for producing diaphragms for acoustic transducers which comprise a core layer comprising poly(meth)acrylimide foam and at least one covering layer is complicated since it is a two-stage process. Furthermore, it is suitable only for covering layers which have a fibrous structure, since volatilization of solvent from the composite of core layer and covering layer is only ensured by these.
Furthermore, it has been found that the decoration films easily become detached from the core layer on prolonged use if they have been applied to a particularly smooth poly(meth)acrylimide layer. Here, it has to be remembered that although the membrane should be configured as a stiff cylinder, this aim can be achieved only incompletely and vibrations and deformations within the membrane are unavoidable. These vibrations can lead to detachment occurring over a prolonged period of time.
In view of the prior art reported and discussed here, it is an object of the present invention to provide a process for producing diaphragms for electroacoustic transducers which comprise a core layer comprising poly(meth)acrylimide foam and at least one covering layer. This process should be very simple to carry out.
A further object of the invention is to provide a process for producing a diaphragm for electroacoustic transducers in which particularly short cycle times can be achieved.
A further object of the invention is to provide diaphragms for electroacoustic transducers which comprise a core layer comprising poly(meth)acrylimide foam and at least one covering layer, in which diaphragms the abovementioned detachment problems of the covering layer are reduced or eliminated.
These objects and further objects which are not explicitly mentioned but can be derived in a self-evident manner from the relationships discussed here or necessarily follow therefrom are achieved by a process for producing a diaphragm for electroacoustic transducers which comprise a core layer comprising a poly(meth)acrylimide foam and at least one covering layer and have all the features of the independent process claim 1.
Advantageous embodiments of the process of the invention are claimed in the subordinate claims dependent on claim 1. In respect of the diaphragm for electroacoustic transducers, the subject matter of claim 8 provides a solution to the object of the invention.
A process for producing a diaphragm for electroacoustic transducers which comprises a core layer comprising poly(meth)acrylimide foam and at least one covering layer, by means of which the covering layer is applied particularly firmly to the core layer, comprises laminating the covering layer with the core layer under a pressure of xe2x89xa70.4 MPa at a temperature of xe2x89xa7160xc2x0 C., at the same time compacting at least the side of the core layer which is in contact with the covering layer and subsequently cooling the resulting composite to a temperature below 80xc2x0 C. before reducing the pressure to ambient pressure.
Secondly, a diaphragm for electroacoustic transducers which comprises a core layer comprising poly(meth)acrylimide foam and at least one covering layer and has a peel strength of xe2x89xa710 Nmm/mm, a modulus of elasticity of xe2x89xa750 MPa and a flexural strength of xe2x89xa72 MPa provides a diaphragm whose covering layer does not become detached from the core layer even after prolonged use.
The following advantages, in particular, are achieved by the measures according to the invention:
Combination of compaction and lamination in one process step.
The process of the invention makes very short cycle times possible.
The covering layer is applied particularly firmly to the core layer by means of the process of the invention.
In the process of the invention, it is also possible to use covering layers which do not have a fibrous structure.
A desired strength of the component can be set within a wide range via the degree of compaction of the core layer in combination with selection of the covering layer.
The process of the invention is characterized in that the covering layer is laminated with the core layer under a pressure of xe2x89xa70.4 MPa at a temperature of xe2x89xa7160xc2x0 C., at the same time compacting at least the side of the core layer which is in contact with the covering layer is compacted and the resulting composite is subsequently cooled to a temperature below 80xc2x0 C. before the pressure is reduced to ambient pressure. This can generally be achieved by hot pressing. These methods are widely known in the prior art, and specific embodiments such as double built presses, SMC presses and GMT presses are also encompassed by the invention. Preference is given to using spacers, known as stops, in the pressing procedure. These make it easier to set a desired degree of compaction of the core layer, but the invention is not restricted to such a procedure.